[bug#36351] [PATCH 06/10] derivations: 'derivation' preserves pointer equality.

["Ludovic Courtès" <ludo@gnu.org>]

* guix/derivations.scm (derivation): Check if FILE is already in
%DERIVATION-CACHE and return it if it is.
---
 guix/derivations.scm | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/guix/derivations.scm b/guix/derivations.scm
index 5c568f223b..403e86749b 100644
--- a/guix/derivations.scm
+++ b/guix/derivations.scm
@@ -873,8 +873,12 @@ derivation.  It is kept as-is, uninterpreted, in the derivation."
                                     (append (map derivation-input-path inputs)
                                             sources)))
            (drv* (set-field drv (derivation-file-name) file)))
-      (hash-set! %derivation-cache file drv*)
-      drv*)))
+      ;; Preserve pointer equality.  This improves the performance of
+      ;; 'eq?'-memoization on derivations.
+      (or (hash-ref %derivation-cache file)
+          (begin
+            (hash-set! %derivation-cache file drv*)
+            drv*)))))
 
 (define (invalidate-derivation-caches!)
   "Invalidate internal derivation caches.  This is mostly useful for
-- 
2.22.0